1. Field of the Invention
The present invention generally relates to I/O connections (including power connections) of circuit modules and, more particularly, to improved connections to I/O pins in modular circuits which are subject to mechanical and thermal stressing.
2. Description of the Prior Art
Modular circuit designs have been known and employed in complex electronic devices for numerous applications. Modular circuit designs allow substantial convenience in packaging and manufacture as well as for repair and implementation of upgraded or improved modules which may improve the function of the overall apparatus as the improved modules become available. However, to allow realization of such advantages by modular construction of electronic apparatus, inputs and outputs, including power connections, must be made by a more-or-less readily detachable mechanical connections.
In recent years, many applications for complex electronic devices have involved particularly severe environmental circumstances. Avionics is exemplary of applications in which the structure of electronic devices may be subjected to extreme temperature excursions, acceleration, vibration and possibly even stresses transmitted from the airframe of the aircraft. Thermal excursions can also generate stresses internally of the electronic device and/or modular circuits included therein. Such stresses are, of course, carried by the structures by which the various components of the electronic device are supported, regardless of the source of the stresses. When the structures which support a component or module also make electrical connections thereto, the quality of the electrical connection may be compromised in some manner. Pin structures are particularly subject to problems of this type since they, in essence, constitute cantilevered structures which can concentrate forces at the base or other support of the pin.
In particular, flexure of pin connections which are electrically attached to a circuit substrate by soldering tends to cause cracking of the solder connection even when the pin is otherwise supported by the substrate and/or a reinforcing header to which the circuit substrate may be attached. Cracking of solder joints is particularly critical in regard to reliability of electrical apparatus since it is a progressive process which increases resistance and the potential for heating within the connection and, in severe cases could result in solder reflow including the formation of voids. Further, electrical conduction across a crack may be possible under test conditions but may result in intermittent connection or disconnection when the electronic device is placed in service, engendering noise and other signal anomalies.
Accordingly, numerous connection designs and bonding techniques have been attempted to increase reliability of electrical connections which must also carry mechanical loads. For example, welding, brazing and ultrasonic bonding are known but are expensive, generally inconvenient and require significantly more costly specialized equipment to perform. Such equipment generally is of limited throughput and can significantly reduce manufacturing productivity. More permanent connection techniques can also impede circuit or module repair unless other similarly specialized techniques, tooling and equipment are employed, of which U.S. Pat. No. 4,991,286 is exemplary.
Various connection designs are also known which often seek to provide redundant connections and/or mechanical forces which are intended to maintain electrical contact while accommodating some degree of flexure. For example, U.S. Pat. No. 5,514,839 provides for welding of a pin to metallization on a flexible circuit board in which the metallization also forms tabs which bear against the pin for welding thereto. However, this structure requires substantial pin height above the metallization of the circuit board whereas very low (e.g. 0.020 inches) pin protrusion is characteristic of modern modular circuit designs. An additional process step by which the tabs are formed into a desired shape is also required. (The same is true of U.S. Pat. No. 5,389,743 for increasing connection security but does not provide for pin mounting.) Low pin protrusion or header profile height as well as miniaturization of circuit modules limits the space and length available to accommodate pin flexure. U.S. Pat. No. 5,311,400 is directed to reduction of header profile height but uses discrete insulated wires for connections to pins.
Accordingly, it is seen that known arrangements for improving reliability of electrical connections are not readily applicable to low pin protrusion height which is desirable in modern modular circuits. Likewise, known arrangements are not generally simple or inexpensive and cannot be easily implemented in a modular circuit by solder bonding processes.